Partial fingerprint scanners are becoming popular for a wide variety of security applications. In contrast to “all at once” fingerprint scanners, which capture an image of an entire fingerprint at the same time, partial fingerprint sensing devices use a sensing area that is smaller than the fingerprint area to be imaged. By imaging only a portion of a fingerprint at any given time, the size and cost of a partial fingerprint sensor can be made considerably smaller and cheaper than that of a full fingerprint sensor. However to capture a full fingerprint image, the user must move his finger and “swipe” it across the sensing zone of the partial finger print sensor.
Various types of partial fingerprint readers exist. Some work by optical means, some by pressure sensor means, and others by capacitance sensing means or radiofrequency sensing means.
For example, one common configuration used for a fingerprint sensor is a one or two dimensional array of CCD (charge coupled devices) or C-MOS circuit sensor elements (pixels). These components are embedded in a sensing surface to form a matrix of pressure sensing elements that generate signals in response to pressure applied to the surface by a finger. These signals are read by a processor and used to reconstruct the fingerprint of a user and to verify identification.
Other devices include one or two dimensional arrays of optical sensors that read light reflected off of a person's finger and onto an array of optical detectors. The reflected light is converted to a signal that defines the fingerprint of the finger analyzed and is used to reconstruct the fingerprint and to verify identification.
Many types of partial fingerprint scanners are comprised of linear (1 dimensional) arrays of sensing elements (pixels). These one dimensional sensors create a two dimensional image of a fingerprint through the relative motion of the finger pad relative to the sensor array.
One class of partial fingerprint sensors that are particularly useful for small device applications are deep finger penetrating radio frequency (RF) based sensors. These are described in U.S. Pat. Nos. 7,099,496; 7,146,024; and patent application Ser. Nos. 11/107,682; 11/112,338; 11,243,100; 11/184,464, and the contents of these patents and patent applications are incorporated herein by reference. These types of sensors are commercially produced by Validity Sensors, Inc, San Jose Calif. This class of sensor mounts the sensing elements (usually arranged in a one dimensional array) on a thin, flexible, and environmentally robust support, and the IC used to drive the sensor in a protected location some distance away from the sensing zone. Such sensors are particularly advantageous in applications where small sensor size and sensor robustness are critical.
The Validity fingerprint sensors measure the intensity of electric fields conducted by finger ridges and valleys, such as deep finger penetrating radio frequency (RF) based sensing technology, and use this information to sense and create the fingerprint image. These devices create sensing elements by creating a linear array composed of many miniature excitation electrodes, spaced at a high density, such as a density of approximately 500 electrodes per inch. The tips of these electrodes are separated from a single sensing electrode by a small sensor gap. The electrodes are electrically excited in a progressive scan pattern and the ridges and valleys of a finger pad alter the electrical properties (usually the capacitive properties) of the excitation electrode—sensing electrode interaction, and this in turn creates a detectable electrical signal. The electrodes and sensors are mounted on thin flexible printed circuit support, and these electrodes and sensors are usually excited and the sensor read by an integrated circuit chip (scanner chip, driver chip, scan IC) designed for this purpose. The end result is to create a one dimensional “image” of the portion of the finger pad immediately over the electrode array and sensor junction.
As the finger surface is moved across the sensor, portions of the fingerprint are sensed and captured by the device's one dimensional scanner, creating an array of one dimensional images indexed by order of data acquisition, and/or alternatively annotated with additional time and/or finger pad location information. Circuitry, such as a computer processor or microprocessor, then creates a full two-dimensional fingerprint image by creating a mosaic of these one dimensional partial fingerprint images.
Often the processor will then compare this recreated two dimensional full fingerprint, usually stored in working memory, with an authorized fingerprint stored in a fingerprint recognition memory, and determine if there is a match or not. Software to fingerprint matching is disclosed in U.S. Pat. Nos. 7,020,591 and 7,194,392 by Wei et. al., and is commercially available from sources such as Cogent systems, Inc., South Pasadena, Calif.
If the scanned fingerprint matches the record of an authorized user, the processor then usually unlocks a secure area or computer system and allows the user access. This enables various types of sensitive areas and information (financial data, security codes, etc.), to be protected from unauthorized users, yet still be easily accessible to authorized users.
The main drawback of partial fingerprint sensors is that in order to obtain a valid fingerprint scan, the user must swipe his or her finger across the sensor surface in a relatively uniform manner. Unfortunately, due to various human factors issues, this usually isn't possible. In the real world, users will not swipe their fingers with a constant speed. Some will swipe more quickly than others, some may swipe at non-uniform speeds, and some may stop partially through a scan, and then resume. In order to account for this type of variation, modern partial fingerprint sensors often incorporate finger position sensors to determine, relative to the fingerprint sensor, how the overall finger position and speed varies during a finger swipe.
One type of finger position indicator, represented by U.S. Pat. No. 7,146,024, and application Ser. Nos. 11/112,338 and 11/107,682 (the contents of which are incorporated herein by reference) detects relative finger position using a long array of electrical drive plate sensors. These plates sense the bulk of a finger (rather than the fine details of the fingerprint ridges), and thus sense the relative position of the finger relative to the linear array used for fingerprint sensing. A second type of fingerprint position indicator, represented by U.S. patent Ser. No. 11/184,464 (the contents of which are incorporated herein by reference), uses two linear partial fingerprint sensors, located about 400 microns apart. The two linear sensors use the slight timing differences that occur when a fingerprint swipe first hits one sensor and then the other sensor to detect when a fingerprint edge passes over the sensors. This technique can also detect relative speed of passage over the two partial sensors. This type of information can be used to deduce overall finger location during the course of a fingerprint swipe.
In either case, once finger position is known, each of the one-dimensional partial fingerprint images can then be annotated with additional (and optional) time data (time stamp) or finger (finger tip, finger pad, fingerprint location) location data (location stamp). This optional annotation information, which supplements the “order of data acquisition” that would normally be used to keep track of the multiple stored partial fingerprint images in memory, can be used to help to correct distortions (artifacts) when the various one dimensional partial images are assembled into a full two dimensional fingerprint image.
For example, if the user momentarily stops moving the finger during the finger swipe, the system will generate a series of nearly identical partial (one dimensional) fingerprint images. These images will have different orders of acquisition, and differing time stamps, which could confuse a processor when it attempts to create a correct two dimensional full fingerprint image. However if the fingerprint scanner also has a finger position sensor, the finger location data stamp associated with these nearly identical one dimensional partial fingerprint images will provide evidence that the finger stopped because the finger location data linked to these various one-dimensional partial fingerprint images will be almost the same. The computer processor that reassembles the partial fingerprint images into the complete fingerprint image can be instructed or programmed to also analyze the finger position (location) data, and perform appropriate image corrections when the location data shows that the finger paused during a scan.
Fingers and fingertips are not totally rigid, however. Finger prints rest on a surface layer of skin, which in turn rests over a bed of non-rigid tissue, which in turn rests on bone. Finger tips are thus flexible and deformable, and also slightly sticky due to the innate property of skin, sweat glands, and the high surface area imparted by fingerprint ridges. Thus when a fingertip slides over a surface, there is a tendency for portions of the fingertip pad to momentarily adhere to the surface, while the top of the fingertip continues its motion. This is called “stiction”. Eventually (usually within a fraction of a second), further motion of the finger tip and bone creates enough force to disrupt the momentary adhesion (stiction) between the finger pad skin and the surface, and the motion of the finger pad skin continues. However if, during this brief period of arrest (stiction), the fingertip skin is arrested over a partial fingerprint scanner, a short series of nearly identical repeated partial fingerprint images will be collected. These nearly identical images will have different order of acquisition indexes, and different time stamps. Because the tip of the finger may have continued in motion while the skin was arrested over the sensor, the nearly identical images may also have different finger position (location) stamps. However, these different finger location stamps are misleading because during the stiction event, the finger pad skin itself may not have moved.
Thus when a processor connected to the partial fingerprint sensor attempts to reassemble the series of one dimensional images into a complete two dimensional fingerprint, a distorted image will result. This distorted data is undesirable because it can confound security authorization schemes that require high quality, undistorted, fingerprint images. Efficient methods to correct for such fingerprint “stiction” artifacts are thus desirable.